btrfs: stop using run_check in _run_btrfs_balance_start

btrfs: stop using run_check in _run_btrfs_balance_start

The use of run_check() immediately stops a test because it calls the
_fail() function when execution of its argument fails. This is
generally not encouraged in fstests as it prevents a test from
detecting further problems after that failure.

Since the next patch in this series updates other tests to use
_run_btrfs_balance_start() for which a failure to run balance can be
expected (btrfs/187 for example), remove the use of run_check() from
_run_btrfs_balance_start(). Existing tests that use
_run_btrfs_balance_start() now redirect standard output to the
test's .full file for debugging purposes. In case balance fails the
tests will fail due to unexpected output from the standard error.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>